1. Field of the Invention
The present invention relates generally to bone fixation screws and, more particularly, to a bone fixation screw of a unique construction designed to inhibit loosening which customarily occurs as the bone relaxes subsequent to fixation.
2. Description of the Prior Art
Currently, bone screws are used to affix bone fragments in direct apposition after proper alignment to promote healing. Upon fracture of a bone, the body's response is to stabilize and heal the fragments in a functional orientation. When a fragment or fragments are displaced in a non-functional or non-anatomical orientation, non-union or malunion can occur. In this instance, open reduction of the fracture may be necessary.
In open reduction, a surgical opening is made, the fracture fragments are realigned, and plates and screws, wire, or other hardware are added to secure the fracture fragments to the bones from whence they came. When a screw is used, whether to secure two fragments together or to secure a plate to the bone, and the screw is tightened, initially, tension in the screw is very high, and holds the fragments together. However, bone is a viscoelastic material and undergoes a phenomenon known as stress relaxation immediately after torque has been applied to the screw. The stress relaxation response is quite pronounced and causes immediate and rapid reduction in the screw tension and, hence, the force holding the fragments together. Typical constructions of known orthopedic fasteners are disclosed in U.S. Pat. No. 4,537,185 to Stednitz, U.S. Pat. No. 4,711,232 to Fisher et al, U.S. Pat. No. 4,227,518 to Aginsky, U.S. Pat. No. 3,554,193 to Constantinou, and U.S. Pat. No. 3,051,169 to Grath. In each of these instances, a standard rigid screw is employed which is certain to become loosened relatively rapidly due to the stress relaxation phenomenon previously mentioned.
Of course, the same situation exists when adjoining bone fragments resulting from a fracture are re-joined before the healing process has begun. Bone screws may also be used to provide allograft or auto-graft fixation during revision surgery or to provide secondary fixation of an orthopedic component, such as an acetabular cup, tibial tray, glenoid component, ulnar component, or the like, to the prepared bony structure at the time of total joint replacement surgery. Bone screws may also be used to fix bone fragments in a traumatic case in which screw removal may be unlikely. In each of these events, however, while the screws provide an initial fixation, the viscoelastic material properties of bone will not maintain the compressive loads generated by the screws. As a result, within a relatively short time, the initial fixation may be greatly compromised.